Dave's Archives

When my housemate told me of his scheme for constructing a physics simulation, I did the only decent thing I could. I stole his idea. I turned to my copy of Serway and Beichner (a ~1600 page physics book that I keep under my pillow for just such emergencies) and sat down to work out how to get 2D circles to bounce off each other in the expected manner. Not that anyone has actually seen two-dimensional circles bouncing off each other in everyday life, what with the universe being three-dimensional and everything, but somehow we still have a conception of what it would be like if it were possible.

This took a surprising amount of math (much of which was admittedly due to a series of mistakes). Collisions must preserve overall momentum and energy, and the direction of acceleration must be determined by the angle of collision. And it's nice if you can remember the quadratic formula, and work out which of the two answers it gives you is the right one in this circumstance.

Eventually I got it working, using a combination of C++, KDevelop, CMake, SDL and OpenGL, all of which I was using for the first time in years, or in some cases ever. My housemate valiantly tried to suggest the use of autopointers, but I was all learned out at that point. So, having triumphed in two dimensions where others have merely succeeded years or decades ago in three dimensions, I now have a little black window containing 20 or so blue circles of various sizes (and virtual masses) bouncing around and hitting each other.

The algorithm assumes for the moment that a given circle can collide with at most one other circle in every discrete time unit. When multiple simultaneous collisions occur, at least two circles become entwined - partially overlapping. They constantly "collide", in each and every time unit, each time alternating direction with respect to each other but never gaining sufficient velocity to escape. They each just vibrate back and forth. The net effect is that they become a combined object, which exhibits angular momentum. The two balls spin around each other, with the "heavier" one making less pronounced motions than the "lighter" one. And I haven't even tried to model angular momentum yet.

The next step will be allowing for multiple collisions with the same object in the same time unit, which should theoretically stop this from happening. But I might just have to preserve the current version.

Sadly, due to seemingly increasingly frequent downtime at bur.st, I've shifted Dave's Archives over to a paid service - Jumba. In the process, I've discovered to my amusement and mild shock that one doesn't actually need to use the command line anymore in order to set up a website on a Linux machine, even if you have PHP web applications with database backends. Jumba (and, I'm informed, most other web hosting services that run Linux) lets you use a web-based interface called cPanel to manipulate just about everything you could possibly want to manipulate. It even has a web-based file manager built in, which actually works in Konqueror. It even has an add-on called Fantastico, which will install and configure web applications (like WordPress) with little more than a couple of button clicks on your part. I've shifted the entire site - database, theme and other miscellaneous settings - without even glimpsing the command line.

I suppose a few seasoned web developers are raising their eyebrows wondering how I managed to find a rock large enough to hide under for the last five years, or for however long this sort of thing has been going on. This is a new thing for me. My brain associates "using Linux" with either GNOME, KDE or the command-line, the first two of which are (generally) irrelevant if you're accessing the computer remotely. Nostalgia be damned. I have seen the light, and it is good.

I present to you the Frivolous Theorem of Arithmetic:

Almost all natural numbers are very, very, very large.

The user interface of OS X has many things to commend it. Its click-to-focus function is not one of them.

OS X, by design apparently, lacks a click-through capability. As far as I can tell there is no way to enable it. This generally means that when you click on a window to select it, the window itself will be blissfully unaware of the event. If you want to press a button in an unselected window, you must click once to select the window and once again to press the button.

As far as I can tell (and I could have missed something here), this is designed to prevent you activating some random hideous piece of functionality by accidentally clicking off the edge of the selected window, on the window underneath. However, while in certain somewhat undesirable states of mind, this subtle obstacle can become unimaginably irritating.

But surely Dave, can't you just get used to double-clicking to press a button in an unselected window?

But I damn well shouldn't have to! What if you have more than one screen (something that OS X in general handles quite smoothly)? The whole rationale falls to pieces. You have multiple screens so that you can rapidly switch your attention between multiple windows without worrying about the mechanics of the window manager. That's not to say I haven't tried getting used to it, but unfortunately OS X provides only a subtle visual distinction between selected and unselected windows. It's rather counterintuitive to have to modify your actions based on the colour of the buttons in the top-left of the window.

Even if I did get used to it, I'd still be screwed because if you double-click on an unselected window, the window receives a double-click event, not a single-click, and something unwanted is bound to happen. For instance, my text editor (Smultron) lists the currently-open files down the left side of the window. If you single-click on a filename, that file will be displayed. If you double-click, the file will be displayed in a whole new window. That in itself is a perfectly reasonable and useful thing for a text editor to do, IMO, but couple it with OS X's window management and it can become a headache.

What you actually have to do is click once to select the window, wait a prescribed amount of time, and then click again to do what you wanted to do. Gah...

What happens when you set a cron job to run every fifteen minutes, generating mail, and then leave the computer in the hands of someone who doesn't know what a cron job is? 266MB and 54500 messages stuck in a mail directory that would never be read. Today it was turned off, and all trace of its existence (bar this post) has been wiped out. It had been running since January 10, 2006. It could have gone on much, much longer.

As a geek I set these things up as a convenience, but somehow I've never really trusted the technology enough to believe it would run for more than a few weeks without my intervention one way or another. I wonder how many other abandoned, forgotten cron jobs are out there, like the vestigial body parts of a species that has evolved away from them...